Morphology and mechanical performance of polystyrene/polyethylene composites prepared in supercritical carbon dioxide

Polystyrene/polyethylene composites have been prepared by the heterogeneous radical polymerization of styrene within supercritical carbon dioxide-swollen high density polyethylene (HDPE) substrates. Composition of the composites can be controlled with reaction time and initial ratio of styrene to HDPE. The polystyrene produced within the substrate is of high molecular weight. Differential scanning calorimetry and wide-angle X-ray diffraction indicate that the crystalline portion of the HDPE substrate is unaffected by the procedure used in this investigation. Scanning electron microscopy indicates that the polystyrene resides in the noncrystalline domains and permeates throughout the spherulitic structure of the HDPE substrates. This morphology is very different from the morphology of polystyrene/polyethylene blends produced by conventional melt/mixing techniques. The strengthening of the spherulitic structure of HDPE produces an efficient enhancement in the modulus of the overall composite. The tensile strengths of the composites are dramatically enhanced over conventionally produced blends. The addition of brittle polystyrene to extremely tough HDPE substrates decreases the overall fracture toughnesses of the composites.